Vintage Synth Repair

From: Scott Nordlund <gsn10@hotmail.com>
To: vintagesynthrepair@yahoogroups.com
Sent: Wednesday, September 9, 2009 11:33:48 PM
Subject: [vintagesynthrepair] TX816 power supply


> Hi
>
> I just dusted off my TX816 only to find that turning the power on does nothing. No glowing LEDS anywhere.
>
> It was working when I put it away.
>
> The AC seems to pass through the filter OK so I assume it is the switch mode power supply (dated 1986).
>
> Nothing burnt and no visibly leaking caps. There is a 470u 200V electro cap (C7 for those with cct diagrams) with a slightly bulging top.
>
>
>
> I must confess mucking about in the 240V realm is a bit daunting.
>
> What is the best approach to fixing this?
>
> What are the most likely failure modes in a switch mode supply?
>
> Can I leave the power supply disconnected from the other boards to test it or does it need to be loaded to function correctly?


Switched-mode power supplies can fail in a lot of ways, some of which can be very bad (overvoltage, chain reaction component failures..).  They often need to be loaded to work properly, and depending on the design can be damaged if operated with no load.  I would help to have a dummy load since you wouldn't have to put your TF1s at risk.

Most often you'll find that it's sort of "ticking".  A bootstrap resistor starts the circuit, but can't supply enough current to keep it running, so if it's not working for some reason, it will repeatedly cycle through the start up sequence.  You can often do enough testing in this state to diagnose the problem.

You should note also that the primary side of the circuit (which may include all of the control circuitry) is likely just rectified line voltage- even the "ground" here may be hot with respect to ground, so you should either use an isolation transformer, or just be very careful.  Connecting an oscilloscope probe's ground lead to this may cause fireworks, so watch what you're doing.



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------------------------------------

Yahoo! Groups Links

>
> Try this:
> 
> http://www.capetronics.com/switching_power_supplies.htm#smpstsps
> 
> 
> 
> 
> 
> 
> ________________________________
> From: Scott Nordlund <gsn10@...>
> To: vintagesynthrepair@yahoogroups.com
> Sent: Wednesday, September 9, 2009 11:33:48 PM
> Subject: [vintagesynthrepair] TX816 power supply
> 
> 
> > Hi
> >
> > I just dusted off my TX816 only to find that turning the power on does nothing. No glowing LEDS anywhere.
> >
> > It was working when I put it away.
> >
> > The AC seems to pass through the filter OK so I assume it is the switch mode power supply (dated 1986).
> >
> > Nothing burnt and no visibly leaking caps. There is a 470u 200V electro cap (C7 for those with cct diagrams) with a slightly bulging top.
> >
> >
> >
> > I must confess mucking about in the 240V realm is a bit daunting.
> >
> > What is the best approach to fixing this?
> >
> > What are the most likely failure modes in a switch mode supply?
> >
> > Can I leave the power supply disconnected from the other boards to test it or does it need to be loaded to function correctly?
> 
> 
> Switched-mode power supplies can fail in a lot of ways, some of which can be very bad (overvoltage, chain reaction component failures..).  They often need to be loaded to work properly, and depending on the design can be damaged if operated with no load.  I would help to have a dummy load since you wouldn't have to put your TF1s at risk.
> 
> Most often you'll find that it's sort of "ticking".  A bootstrap resistor starts the circuit, but can't supply enough current to keep it running, so if it's not working for some reason, it will repeatedly cycle through the start up sequence.  You can often do enough testing in this state to diagnose the problem.
> 
> You should note also that the primary side of the circuit (which may include all of the control circuitry) is likely just rectified line voltage- even the "ground" here may be hot with respect to ground, so you should either use an isolation transformer, or just be very careful.  Connecting an oscilloscope probe's ground lead to this may cause fireworks, so watch what you're doing.
> 
> 
> 
> _________________________________________________________________
> Windows Live: Make it easier for your friends to see what you’re up to on Facebook.
> http://windowslive.com/Campaign/SocialNetworking?ocid=PID23285::T:WLMTAGL:ON:WL:en-US:SI_SB_facebook:082009
> 
> ------------------------------------
> 
> Yahoo! Groups Links
>